The present invention relates to a synchronous filter with switched capacitances. It applies to the filtering of a signal having a useful component associated with a significant background noise.
The filter according to the invention is of the band-pass type with a narrow band centered on the frequency of the useful components of the input signal. The center frequency of the pass band is determined by the frequency of the clock pulses controlling the switching of the capacitances. This filter has N pairs of channels, each channel having at least one capacitance to be switched.
The invention applies to the selection of a periodic or cyclic signal mixed with another signal, harmonic analysis, telecommunications, processing the signal in nuclear magnetic resonance, servocontrols, etc.
It is known that highly selective band-pass filters can be very easily produced by using structures with N switched paths, each having a resistor associated with a capacitor and switches, as will be shown in greater detail hereinafter. These structures can be reduced to a circuit equivalent to N paths, in which each path comprises at least one capacitor, associated with two switches, all the paths being connected to a common input resistor. This equivalent circuit is often called a filter with N paths with capacitance switching. The main advantage of this type of filter is that it has a "comb" response, a pass band with a regulatable width and can be frequency tuned due to the pulses of an external standard clock. In microelectronic technology, the latter property makes it possible to produce filters having an integratable structure. Filters with external frequency standards are much less sensitive than other filters to variations in the values of the elements forming the same.
The external regulating of their tuning frequency, as well as their synchronization by an external signal can be performed in a simple manner. The center frequency of the main tooth of the response curve of such "comb" filters is defined by the frequency of the pulses of the external clock.
Obtaining a very narrow pass band with a filter of this type requires an increase in the number N of paths of the filter. The increase in the number of paths of the comb filter simultaneously leads to an increase in the external control clock frequency which, for a given technology, can lead to a limitation of the high frequency of the filter.
It results from these observations that a switching filter has a pass band, whose narrowness is limited by the frequency of the switching signals.
Another objective which is sought in filters is to reject any parasitic signal or run-out, without causing any phase displacement between the input signal of said filter and the output signal thereof.
Most known filters introduce a phase displacement between the input signal and the output signal. However, there are so called synchronous switched capacitance filters, in which the synchronization is defined by the frequency of the control clock pulses of the switching operations of the filter paths. These known filters are more particularly described in IEEE JOURNAL OF SOLID STATE CIRCUITS, Vol SC18, No 6, December 1983, pp 753-761. They have the main feature of having a plurality of pass bands instead of one such band and this is very prejudicial in certain applications. Thus, if one of the pass bands is centered on a run-out, the latter is transmitted, which can be highly prejudicial in certain applications. Synchronous filters do not have the same equivalent pass band band width with respect to a periodic signal and a random signal, as is apparent from the book by Robert Miquel, entitled "Le filtrage numerique", editions Editest, February 1985, p 143.
In synchronous filtering, it is often useful to obtain an absence of response of the filter at a zero frequency and at even harmonics of the input signal. It is also useful to obtain a significant attenuation of the odd harmonics. These known switching synchronous filters unfortunately do not make it make it possible to meet all these requirements.
Known filters making it possible to partly meet the requirements regarding phase, elimination of even harmonics, attenuation of odd harmonics and elimination of run-out are described in the following journals :
The JOURNAL AEG-BAND 25, 1971, No 4, pp 173 to 150 describes switching filters with N paths of the stop--go type. This type of filter suffers from the main disadvantage of having several pass bands and of not making it possible to obtain a signal filtered in phase with the input signal. PA1 IEEE JOURNAL OF SOLID STATE CIRCUITS, VOL SC15, No 3, June 1980, pp 301 to 305 describes a switching synchronous filter using switched capacitances, but whose structure does not make it possible to meet the phase and harmonic elimination requirements. PA1 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS, VOL CAS 29, No 8, August 1982, pp 557 to 572 describes a switching filter with N paths and switched capacitances. This journal reveals the defects mentioned hereinbefore for this type of filter.